population swings by one or a few species. Often this is best achieved by restoring native species. Although ecologists no longer equate diversity directly with ecosystem stability, biotic diversity, often stated in terms of species or genetic diversity, is still an important measure of ecosystem quality.
Many so-called lake restoration projects really are only mitigation and management efforts to rid a lake, by whatever means, of some nuisance. Criteria for selection of the procedure are based primarily on cost and effectiveness relative to the specific target (e.g., elimination of a nuisance organism). Some procedures that rank highly based on those criteria fare poorly when evaluated more broadly in terms of total ecosystem restoration. For example, chemical herbicides are commonly used to control rampant macrophyte growths in littoral zones of lakes and, if applied properly, can achieve the goal of removing the nuisance, at least temporarily. However, the dead plant material may release a substantial load of nutrients to the water column, inducing excessive algal growth (substituting one problem for another), or the herbicide may adversely affect nontarget organisms. Most important, herbicide treatments and similar palliatives do not get at the underlying causes of a problem. In the example given, once the herbicide disappears, the macrophyte problem probably will return.
As stated above, the goal of most lake restoration projects is to remove a specific problem — a nuisance organism, excess chemical(s), or unwanted physical condition — and make the lake more desirable (based on human-centered criteria) and more usable for specific human purposes. The success of a restoration project is (and must be) evaluated according to the extent to which these human-oriented goals are met (e.g., Was the fishery restored? Was swimming improved?). Chemical measures of water quality (and associated numerical criteria for specific chemical species) may be used to measure the success of restoration when the problem has a simple cause (e.g., presence of a toxic chemical), but more typically the causes and symptoms of degradation are much more complicated. Quantitative measurements of improvements in recreational and aesthetic attributes are notoriously difficult to obtain, however, and therefore success commonly is measured in terms of quantifiable water quality characteristics such as Secchi disk transparency (a measure of water clarity) and concentration of chlorophyll a (a measure of algal biomass) that are loosely related to recreational and aesthetic conditions.